1. Field of the Invention
The present invention relates to ion attachment mass spectrometry and a mass spectrometer for measuring, in a fragment-free state, an object to be measured contained in a sample, and particularly to ion attachment mass spectrometry and a mass spectrometer for measuring, in a fragment-free state, an object to be measured contained in a sample that discharges a reducing gas or a sample that contains a reducing gas.
2. Description of the Related Art
In the mass spectrometry, the molecule of an object to be measured contained in a sample is ionized, and after that, ions are fractionated according to mass (mass number) by an electromagnetic technique to measure the intensity of each ion. The ionization part of the front half is called the ionization section (ionization apparatus), and the mass fractionation part of the latter half is called the mass spectrometry section (mass spectrometer). The mass spectrometry occupies a position of representative technique of instrumental analysis methods because of high sensitivity, accuracy thereof, or the like, and is utilized in such a wide range of fields as material development, product inspection, environmental research and bio study. In many of these, the spectrometer is used in a state connected to such a component separation device such as a gas chromatograph (GC), and there are such problems that the component separation requires the purification of a sample, and that such a long time as several ten minutes is necessary by the time the component separation is completed. In addition, there are such problems that an object to be measured contained in a sample may be changed in quality or lost in the component separation, deep knowledge and experience are necessary for the component separation, or the like.
Consequently, for the purpose of promptness, ease and high accuracy, a “direct measuring method,” in which measurement is performed by a mass spectrometer alone without being connected to a component separation device, is also used.
There are some kinds of ionization apparatuses for use in the “direct measuring method”, having a principle and structure significantly different from one another, and an ion attachment mass spectrometer has such advantage as capable of performing mass spectrometry of a gas to be detected without the generation of dissociation. Previously, Hodge (Analytical Chemistry vol. 48, No. 6, P825 (1976)), Bombick (Analytical Chemistry vol. 56, No. 3, P396 (1984), Fujii (Analytical Chemistry vol. 61, No. 9, P1026 (1989) and Japanese Patent Application Laid-Open No. 6-11485 reported on the ion attachment mass spectrometer.
FIG. 3 shows a conventional ion attachment mass spectrometer, which measures the mass number of an object to be measured contained in a sample by heating a solid sample or a liquid sample.
In FIG. 3, an emitter/ionization chamber 100 in which an emitter 107 is disposed and a sample vaporizing chamber 101 in which a sample 105 is disposed are arranged in a first chamber 130, and a mass spectrometer 160 is arranged in a second chamber 140. The pressure in the first and second chambers 130 and 140 is reduced by a vacuum pump 150. Accordingly, all of the emitter 107, emitter/ionization chamber 100, sample vaporizing chamber 101 and mass spectrometer 160 exist in an atmosphere of reduced pressure lower than atmospheric pressure (in vacuum).
The emitter 107 constituted of alumina silicate containing an oxide of an alkali metal such as lithium is heated to generate a metal ion 108 having a positive charge such as Li+. That is, the emitter 107 is a sintered body in which an oxide, a carbonate or a salt of an alkali metal (such as Li) is incorporated into alumina silicate (a eutectic body of aluminum oxide and silicon oxide). When the emitter is heated to about 600° C. to 800° C. in an atmosphere of reduced pressure, it generates a positively charged alkali metal ion (the metal ion 108) such as Li+ from the surface thereof.
The sample vaporizing chamber 101 is connected to the emitter/ionization chamber 100.
To the sample vaporizing chamber 101, a solid sample or a liquid sample (hereinafter, referred to as a solid/liquid sample) is inserted with a probe (not shown) from the outside, and a solid/liquid sample 105 disposed at the tip of the probe is heated with a heater (not shown). The solid/liquid sample 105 is vaporized (gasified), and, into the inside of the sample vaporizing chamber 101, a neutral gaseous molecule 106 of the solid/liquid sample 105 is discharged as a gas to be detected and introduced into the emitter/ionization chamber 100.
Consequently, neutral gaseous molecules 106 are ionized in the emitter/ionization chamber 100 to become ions.
Eventually, generated ions are given force from an electric field and are transmitted from the emitter/ionization chamber 100 to a mass spectrometer 160. Ions are fractionated depending on mass by the mass spectrometer 160 and detected.
Here, the metal ion 108 attaches to the neutral gaseous molecule 106 at a position where unevenness of charge exists, and a molecule to which the metal ion 108 is attached (an ion-attached molecule 109) becomes an ion having a positive charge as a whole. Since the attachment energy (energy for the attachment, which becomes an excess energy after the attachment) is very small, the neutral gaseous molecule 106 does not disintegrate and thus, the ion-attached molecule 109 becomes an ionized molecular ion while maintaining the original molecular shape.
However, after the attachment of the metal ion 108 to the neutral gaseous molecule 106, if the ion-attached molecule 109 is left as it is (kept in the state of holding the excess energy), the excess energy cuts the bond between the metal ion 108 and the neutral gaseous molecule 106. Then, the metal ion 108 is separated from the neutral gaseous molecule 106, which returns to the original neutral gaseous molecule 106. Therefore, such gas as N2 (nitrogen) is introduced from a gas cylinder into the emitter/ionization chamber 100 up to a pressure of about 50 to 100 Pa to allow the gas molecule to collide frequently with the ion-attached molecule 109. As the result, the excess energy held by the ion-attached molecule 109 is transferred to the gas molecule to make the ion-attached molecule 109 stable.
The gas also has another function. That is, the gas is provided with such important function in the ion attachment process as decelerating the metal ion 108 emitted from the emitter 107 by the collision against itself to allow the easy attachment of the ion to the neutral gaseous molecule 106. The gas is referred to as the third body gas.
As a property necessary for the third body gas, there is such condition that the gas has to have a low attachment energy. If the third body gas has a great attachment energy and high sensitivity, the metal ion 108 that is limited in the generation amount attaches to the third body gas that exists in a large amount and is consumed, to thereby reduce the percentage of the attachment to the essential object to be measured (to lower the sensitivity). As shown in FIG. 3, the third body gas cylinder 170 is connected to the emitter/ionization chamber 100 via a laying pipe so that N2 can be introduced into the emitter/ionization chamber 100 as the third body gas.
Incidentally, when an organic gas sample was used as the sample, a long measurement time occasionally led to the gradual decrease (on a week-by-week basis) in the generation amount (emission amount) of the metal ion from the emitter. Regarding the phenomenon that affects greatly the sensitivity and accuracy in the measurement, Japanese Patent Application Laid-Open No. 2002-170518 concludes that the decrease in the emission amount is caused by a gradual covering of the emitter surface with carbon or a high-molecular-weight organic compound. Under the recognition, Japanese Patent Application Laid-Open No. 2002-170518 discloses an invention in which the emission amount of the metal ion is secured by supplying an active gas for removing the organic compound on the emitter surface along with the third body gas to the ionization region, in a configuration using an organic gas sample.
On the other hand, there was a time when the emission amount from the emitter decreased even when the sample was a solid sample or a liquid sample (hereinafter, also referred to as a “solid/liquid sample”). For example, when the sample was resin (plastic), and when an object to be measured contained in the resin such as a “resin additive” added for improving various properties (such as flame resistance or flexibility) of the resin was intended to be measured, the emission amount of the metal ion from the emitter decreased during the measurement. However, in this case, such a phenomenon was found that the emission amount decreases during the heating of the sample, but that, after the completion of the heating, the emission amount gradually (on a second-by-second basis) increases and it largely returns to the initial emission amount in several ten minutes. The phenomenon can not be explained by the reason for the gas sample that it is caused by the emitter being covered with an organic compound, as described in the Japanese Patent Application Laid-Open No. 2002-170518.
The decrease in the emission amount causes the degradation of the detection lower limit, and particularly, the variation of the emission amount on a second-by-second basis is serious, which significantly degrades the quantitative accuracy to thereby halve the value as the mass spectrometer.